A slit illumination system provided in an image forming apparatus of slit exposure type for slit-illumination of an original placed on a document glass table in the image forming apparatus. The slit illumination system is provided with a light source including an elongated exposure lamp to illuminate the original at a slit-illuminated position on the document glass table, a main reflecting mirror portion composed of first and second reflecting mirrors, and an auxiliary reflecting mirror portion composed of third and fourth reflecting mirrors. The first, second, third and fourth reflecting mirrors are respectively shaped to follow a part of first, second, third and fourth ellipsoids which are different in curvature from each other.